Increasing the speed and performance of integrated circuits (“ICs”) typically calls for increasing the density of electronic components on the surface of a semiconductor wafer and increasing the speed at which the IC performs its functions. Increasing component density brings charge-carrying circuit elements closer together, thereby increasing the capacitive coupling (crosstalk) between such circuit elements and delay in the propagation of signals through the conductors. Higher capacitance is detrimental to circuit performance, especially for higher frequency operation as would typically be encountered in telecommunication applications and elsewhere.
One way of reducing capacitive coupling between proximate circuit elements is to reduce the dielectric constant (“k”) of the insulator or insulating material(s) separating the coupled circuit elements by using low-k dielectric materials. The terms “low k dielectric materials” and “low-k dielectrics” are generally used to refer to dielectric materials with a dielectric constant less than that of silicon dioxide, which has a dielectric constant of about 3.9. Note that the term “low-k dielectric” applies to a material that has a low dielectric constant subsequent to processing; the material may have a higher dielectric constant upon deposition or during wafer processing. Although a number of low-k dielectric materials are available, many are incompatible with the oxidizing environments commonly encountered during IC fabrication. Oxidizing environments may be encountered during the ashing process and during curing.
“Ashing” refers to a process of removing photoresist from a substrate. In a typical patterning sequence for an etch process (for example), photoresist is applied to a layer to be etched. The photoresist is then exposed through a mask, which contains the features that define the pattern to be created. The photoresist is then developed. The development process leaves patterned photoresist on the layer and removes the rest of the resist. Next, areas of the substrate not protected by photoresist are etched. Typically, an etch process is chosen that selectively removes the material exposed by the patterned resist while causing acceptably little damage to the resist itself. Finally, the residual photoresist is removed by ashing: that is, placing the wafer in or downstream from a heated, reduced-pressure oxygen-containing or reducing plasma. Typical reducing plasmas include hydrogen, ammonia, or nitrogen/hydrogen gas combination (conventionally termed “forming gas”) plasmas. The oxygen and hydrogen ions and/or radicals of the plasma are highly reactive towards carbon-carbon bonds in the resist, breaking the photoresist into volatile species such as carbon dioxide and water. The gaseous species may then be pumped out.
Ashing with an oxygen-containing plasma provides the highest ash rates, but is problematic when used in conjunction with many low-k dielectric materials. Low-k dielectrics include, for example, carbon-doped oxides, aerogels and xerogels, and mesoporous silica and silicalite films. However, these materials generally rely on a small percentage of Si—O—C or Si—C bonds to render them hydrophobic. During the ashing process, these bonds may be replaced by hydrophilic bonds, leading to absorption of water and an increase in dielectric constant. For example, hydrophobic Si—O—Si—(CH3)3 or Si—O—Si(CH3)2—O—Si bonds in the low-k dielectric material may be replaced by hydrophilic Si—OH bonds as a result of the action of the plasma during the ashing process. In a reducing plasma, they may be replaced by Si—H bonds, which can subsequently convert to Si—OH bonds in the presence of water vapor. If hydrophilic bonds form, the low-k dielectric material may absorb water from the ambient after its removal from the ash chamber, which may increase its dielectric constant to unacceptable levels. Additionally, absorbed water vapor may desorb or “outgas” during subsequent high-temperature and/or low pressure processes, interfering with the process.
One approach to the problem is to use hydrogen- or ammonia-based chemistry rather than oxygen-based chemistry for the ash plasma. Hydrogen- and ammonia-based chemistries are more selective to the photoresist and therefore do not lead to a significant increase in dielectric constant. However, the ash rates of hydrogen- and ammonia-based chemistries are typically significantly lower than the ash rate using oxygen chemistry. For example, in a commercially available ash chamber running non oxygen-based chemistries, the ash rate may be decreased to about 25% or 30% of the ash rate using oxygen chemistry. Further, even these processes have been shown to be at least slightly detrimental to most low-k films.
A second process in which an oxidizing environment may be encountered is a cure step carried out after depositing the film on the substrate. Most low-k films undergo a cure step; that is, they undergo chemical reactions after deposition on the wafer to reduce the dielectric constant, stabilize the film, remove reaction byproducts or sacrificial materials, or any combination of these. Using an oxidizing environment would often be advantageous for the cure step. The oxidizing environment may be provided by increasing the temperature in the presence of oxygen or by providing an oxygen plasma. However, many low-k films are damaged during the cure step.
Therefore, it is desirable to provide a method and apparatus for repairing damage to low-k films after exposure to an oxidizing environment. For plasma photoresist removal, it is desirable to repair the damaged low-k film before it is ever exposed to ambient pressure after ashing. Similarly, for performing a rapid cure step using an oxidizing environment after deposition of a low-k layer, it is desirable to repair the damage to the film before it is exposed to a moisture-containing atmosphere.